(CBS News) -- Boston Marathon bombings suspects Tamarlan and Dzokhar Tsarnaev had a cache of weapons that hints at plans for a followup attack
and investigators now say the brothers may have headed for New York
City if not for the shooting death of Tamerlan in a police standoff on
Thursday night and the eventual capture of Dzokhar Friday night.

Investigators
have new insight from the carjacking victim held hostage by Tamerlan
and Dzokhar Thursday night. The victim speaks little to no English,
according to CBS News special correspondent John Miller, but authorities
pressed him to remember recognizable words from his exchange with the
bombing suspects.

The suspects openly boasted to the victim in English
about their role in the Boston Marathon bombing and carried out the rest
of their exchange in Russian.

John Miller reports that the
victim said, "'the only word I recognized was Manhattan,'" a word which
"tripped a lot" of alarm for authorities, who quickly halted Amtrak
service from Boston to New York, searched the trains.

The tip prompted
the New York Police Department to "flip on its network of license plate
readers at all bridges and tunnels coming into the city," Miller said.
"They loaded all the license plates associated with these guys" to
prevent possible entry into Manhattan.

Surviving suspect Dzhokhar
Tsarnaev continued to answer questions in writing from his hospital bed
Monday night. "He can say about one word at a time," Miller said.

So
far, his account indicates he was "driven by his brother" and that "it
was mostly done online, in terms of radicalization [and] finding
instructions." Investigators say it remains unlikely there is an
international terrorist organization behind the attack or the Tsarnaev's
training.

The investigation also has yielded new insight about
the motive for the robberies and carjacking committed by the suspects on
Thursday night, a bloody night that began when they approached and shot
an MIT police officer, Sean Collier, "in the head, unprovoked," Miller
said.

The
operating theory currently held by investigators looking to explain the
assassination is "that they were short one gun, that the older brother
had a gun, they wanted to get a gun for the younger brother and the
fastest and most efficient way they could think of doing it was a
surprise attack on a cop to take his weapon and go."

But the
suspects failed in that aim, because Officer Collier had a locking
holster and they were unable to remove the gun. "There was apparently an
attempt to yank it," Miller said, "And they couldn't get it and left."